Transverse Instability Studies in a Subscale Chamber

Abstract

High-frequency combustion instabilities result from a coupling between modes of heat release and chamber acoustics. Because the geometric dimensions of the chamber define the relevant acoustic modes, it is particularly difficult to study transverse instabilities in anything other than a full-scale chamber. In this paper, results from experimental studies of a two-dimensional subscale chamber that can nearly replicate full-scale acoustic modes are described. The acoustic modes are primarily driven by injector elements at the endwalls of the rectangular chamber. They were designed to be unstable, based on a vortex-shedding mechanism postulated on earlier results from longitudinal studies. The 27-cm-wide by 19-cm-long chamber produced transverse instabilities with a first width-mode frequency of about 2000 Hz, with no longitudinal mode content in the pressure signal. The chamber comprises a linear array of seven gas-centered swirl coaxial liquid injectors, designed to flow either bipropellants, only oxidizer, or nothing. Eight different flow configurations were tested, yielding peak-to-peak pressure oscillation amplitudes ranging from about 5% to nearly 100% of chamber pressure. The strongest instabilities were observed when endwall injectors flowed both fuel and oxidizer, and amplitudes were reduced as the driving injectors were moved inward. An asymmetric setup demonstrated that the unsteady pressures in warm regions and hot regions have differing pressure amplitudes. To demonstrate the potential use of the chamber for measuring the combustion response to velocity and pressure oscillations, the synchronized measurement and analysis of pressure and flame emission are presented.